Relay interlock circuit



www kmw www Tmuwmmm m M M M M M r f r un NQ mi o o o o o c y M n Ev mm v f f xm 9m Q T S mi N N fmm M fmm fm EIN QN R, E. ANDERSON RELAY INTERLOCK CIRCUIT Filed Aug. s. 1966 sept. 9, 1969 /N VE N TOR R. E. A NDE/PSON f7 f/'\ @JLA A T TOR/VE V United States Patent O 3,466,505 RELAY INTERLOCK CIRCUIT Robert E. Anderson, North Andover, Mass., assignor to Bell Telephone Laboratories, Incorporated, Berkeley Heights, NJ., a corporation of New York Filed Aug. 3, 1966, Ser. No. 569,983 Int. Cl. H01h 47/00, 47/32; H02b 1/24 U.S. Cl. 317-136 1 'Cla'ml ABSTRACT F THE DISCLOSURE This invention relates to relay interlock circuits and, more specilically, to relay interlock circuits which selectively energize from a group of relays any one relay at a time.

A relay interlock circuit is an arrangement of relays together with a power source, associated electronic components, and relay drive circuitry which provides for the specic selection of particular relays until a new relay selection is made or a de-energized command is issued. In certain relay interlock circuit applications it may, for instance, be required to energize but one relay at a time and to maintain such selected relay energized until either the previously selected relay is turned off directly or until another one of a group of relays is selected to be energized.

Although such relay interlock circuits have been devised before, they generally require a large number of relay contacts .for relatively complex interlock arrangements, thereby limiting the useful extension `of such interlock circuit-s to a limited number of relays, Further limi-tations are excessive power consumption for the relay drive circuitry, relatively high voltage requirements of the supply source and, in addition, the possibility of oper-ation of more than one relay at a time during relay transfer periods.

The primary object of the invention is to provide for a relay interlock circuit that may be extended to include any number of relays.

Another object of the invention is to increase the operating eiciency of relay interlock circuits.

A further object of the invention is to assure in a relay interlock circuit the energization of not more than one relay at a time.

To -fullll these objects, the invention provides for a relay interlock circuit having the coils of its relays connected in parallel in the emitter-collector path of a relay drive transistor. Each relay has -associated with it a relay selector switch to complete its power path and to control the -bias on the emitter-base junction of the drive transistor, The activation of a single relay selecter switch energizes the selected relay, if no other relay is as yet energized. If, on the other hand, another relay is already energized, the subsequent activation of a relay selector switch for another relay reduces the bias on the relay drive transistor to such an extent that it is turned olf, thereby de-energizing the previously energized relay. At that ins-tant the rel-ay drive transistor becomes again forward biased, thereby allowing the selected relay to energize. Similarly, if two `relay selector switches are activated at the same time, the

Patented Sept. 9, 1969 relay drive transistor bias is reduced -to such an extent as to turn the transistor otr, there-by preventing the energiz-ation of either of the two relays.

More specifically, in one embodiment of the invention the individual coils of a group of relays are connected in parallel in the emitter-collector path of a relay drive transistor. The power path for each relay is completed eit-her through the contacts of a respective relay selector switch or through its own holding contacts which are connected in parallel with the selector switch. Although the emitterbase junction of the drive transistor is normally forward biased, the closing of respective relay selector switches or the closing of lrespective parallel holding contacts reduces this forward bias on the drive transistor in predetermined steps by connecting resistors in parallel with the emitterbase junction. With lall the relay selector switches open, the emitter-base junction of the relay drive transistor is fully forward biased. However, none of the relays can energize since the emitter-collector path of the transistor remains open. When one of the rel-ay selector switches becomes energized, however, the emitter-collector path of the transistor becomes completed, thereby completing the power path for the selected relay, enabling it Ito energize. The closing of each relay selector switch reduces, in addition, the forward bias on the emitter-base junction of the relay drive transistor. With only a single relay selector switch activated, however, the relay drive transistor remains sufficiently forward biased so that the selected relay may energize. Upon the energization of a relay, its holding contacts close, thereby bypassing the respective selector switch to maintain the selected relay ener- -gized even after the opening of the selector switch.

When the selector switches for more than one relay are activated 'simultaneously several respective shunt resistors are connected in parallel across the emitter-base junction of the relay drive transistor. This changes the bias condition of the transistor to such an extent Ias to turn it off, thereby preventing the energization of `any one of the relays. Similarly, when one or more additional relays are selected while one relay is already energized, the additional shunt resistors thereby switched into the bias circuit also turn off the relay drive transistor to cle-energize the relay previously energized and to prevent the energization of any relay until such time that only a single relay selector switch is activated. A previously selected relay -may also be `de-energized by the momentary interruption of the -relay power path by means of a switch at some convenient point in the power circuit.

The present invention provides therefore for a simple and eicient interlock circuit in which only one relay can be energized yat any one time, and which circuit can be extended to include any desired practical number of relays.

The above and other features of the invention will be more fully understood from the following `detailed description. In the drawing:

FIG. 1 is a schematic diagram of one specific embodiment of the invention requiring two holding contacts per relay; and

FIG. 2 is a schematic diagram of another embodiment of the invention requiring only one holding contact per relay.

In FIG. 1 a relay interlock circuit is shown comprising a power source 10 supplying power to relays 11, 12, and 13 through switch 14, Zener diode 15, and relay drive transistor 16. The power path for respective relays may be completed either through holding contacts 11-1, 12-1, and 13-1 of relays 11, 12, and 13, respectively, or through relay selector switch contacts 17-1, 18-1, and 19-1 which are connected in parallel with respective relay holding contacts. Power source 10 also supplies bias power for the emitter-base junction of relay drive transistor 16 through resistor 20. Resistors 21, 22, and 23, on the other hand, function together either with relay holding contacts 11-2, 12-2, 13-2, or with relay selector switch contacts 172, 18-2, and 19-2, respectively, to reduce the bias on the emitter-base junction of relay drive transistor 16 Whenever either one of the switch contacts or one of the holding contacts is closed.

Before any one of relays 11, 12, or 13 has been selected to be energized, all of the relay selector switches are open and all of the relays are de-energized, so that all of the relay holding contacts are necessarily open. Relay drive transistor 16 is therefore not conducting in its emitter-collector path, even though its emitter-base junction is forward biased by power source through resistor 20. As a result the power drain on source 10` is reduced to a minimum during the standby condition of the interlock circuit.

In the interlock circuit of FIG. 1, any one particular relay may be selected to be energized by closing its corresponding relay selector switch. For instance, when it is desired to energize relay 11, relay selector switch 17 4h-aving switch contacts 17-1 and 17-2 is closed momentarily. The closing of switch contact 17-1 completes the power path for relay 11, vthereby allowing collector current of relay drive transistor 16 to ow to energize relap 11. Switch contact 17-2, on the other hand, completes the connection for resistor 21 in the relay transistor biasing circuit. As soon as relay 11 energizes, its holding contacts 11-1 and 11-2 close and are connected in parallel with switch contacts 17-1 and 17-2, respectively. As la result, holding contact 11-1 keeps relay 11 energized and holding contact 11-2 keeps resistor 21 connected in the biasing circuit even after the release of the momentary-type relay selector switch 17. Whereas before the closing of switch contact -17-2, the entire voltage of source 10 was applied through resistor 20 across the series combination of the emitter` base junction of transistor 16 and Zener diode 15, after the closing of switch contact 17-2 this bias voltage is limited to the voltage appearing across resistor 21. That is, resistors 20 and 21 now act as voltage divider network to lower the forward bias applied to the emitter-base junction of relay drive transistor 16. After relay 11 has been energized, relay contact 11-2 performs the identical function as switch contact 17-2, i.e., it completes the circuit for resistor 21 to maintain the lowered emitter-base yforward bias on transistor 16. As long as only one of the three bias resistors 21, 22, or 23 is connected in the circuit, the voltage applied to the emitter-base junction of relay drive transistor 16 is sufficient to forward bias transistor 16, i.e., the voltage across resistor 21, VR21, is greater than the sum of the voltage drops across the emitter-base junction of transistor 16, VEEN, and Zener diode 15, VZ15, S0 that VR21 VBE16+VZ15 Any one of the other two relays of the circuit of FIG. 1 may be selected in a similar manner by momentarily closing its corresponding relay selector switch. If, for instance, relay 12 were to be energized, relay selector switch 18 would be closed, whereas relay selector switch 19 would be closed in order to energize relay 13. On the other hand, 'any one of relays 11, 12, or 13 that is already energized may be de-energized by interrupting its power path by a momentary opening of normally closed switch 14.

In the case where the relay that has been selected for energization is the first relay to be energized, that is, all of the relays are de-energized prior to the selection of this particular relay, the sequence and operation is identical to the sequence and operation described in connection with relay 11, except that contacts 12-1 and 18-1, and 13-1 and 19-1, function together with resistors 22 and 23, to energize relays 12 and 13, respectively. On the other hand, when relay 13, for instance, is selected to be energized while relay 11 is already energized, the following operating sequence takes place. Because relay 11 is already energized, holding contacts 11h1 and 11-2 are closed. Relay drive transistor 16 is forward biased by the voltage across resistor 21, and transistor 16 is conducting through closed holding contact 11-1 to maintain relay 11 energized. Since it is now desired to energize relay 13 instead of relay 11, two functions are performed in sequence; namely, relay 11 is iirst de-energized and then relay 13 is energized. Both of these operations are accomplished in the proper sequence by closing switch contacts 19-1 and 192 of relay selector switch 19. That is, as soon as relay selector switch contact 19-2 is closed, resistor 23 is connected in parallel with resistor 21, thereby reducing the bias voltage on the emitter-base junction of relay drive transistor 16 to such an extent as to turn off transistor 16 instantaneously. The result is fourfold and occurs in the following sequence: (l) even though relay selector switch contact 19-1 is now closed, thereby completing the power path for the coil of relay 13, relay 13 cannot yet energize because of the off condition of transistor 16; (2) since transistor 16 is turned off, relay 11 becomes de-energized, thereby opening its holding contacts, where the opening of holding contact 11-2 disconnects resistor 21 frorn the biasing network so that (3) relay drive transistor 16 again becomes forward biased and, hence, turned on, which allows (4) the energization of relay 13 through relay selector switch contact 191. Once relay 13 has become energized, holding contacts 13-1 and 13-2 bypass relay selector switch contacts 19-1 and 192 to maintain the relay energized even after switch 19 has been opened again. Relay 13 remains energized until either switch 14 becomes momentarily opened, or until another relay is selected.

Resistors 21, 22, and 23 are chosen to have such resistance Values that when source 10 is applied through resistor 20 across the parallel combination of any two or more of these resistors a voltage is developed across the parallel resistor combination which is smaller than the sum of the voltages across Zener diode 15 and the emitter-base junction of relay drive transistor 16. That is, when at any one time more than two of the bias resistors 21, 22, and 23, are connected in parallel inthe biasing circuit, relay drive transistor 16 is immediately turned off to positively prevent the energzation of more than one relay at any one time. This feature of the invention is particularly valuable in circuit applications in which it is required that never more than one relay be energized at any one time in order to prevent undesired circuit operation or damage to the equipment. The present invention further prevents the inadvertent selection of more than one relay at a time. This result is a necessary consequence of the bias network operation, since the simultaneous selection of two relays at any one time causes two bias resistors to be connected in parallel through their respective relay selector switch contacts which, in turn, reduces the bias on relay drive transistor 16 below the required forward bias level. As a result, no relay can energize as long as more than one relay selector switch is being activated at any one time. As soon, however, as only one relay selector switch remains activated, the relay corresponding to that one relay selector switch energizes.

Another specific embodiment of the invention which requires but one holding contact per relay is illustrated in FIG. 2. The interlock circuit of FIG. 2 comprises a power source 30 supplying power to relays 31, 32, and 33 through switch 34 and relay drive transistor 35. Transistor 35 is of the n-p-n type and is so poled as to allow source 30 to energize respective relays either through holding contacts 31-1, 32,-1, and 33-1 of relays 31, 3-2, and 33, respectively, or through relay selector switches 36, 37, and 38 which are individually connected in parallel with respective relay holding contacts. Power source 30 also supplies power to control transistor 39, which in turn controls the conduction of relay drive transistor 35. Control transistor 39 is of the p-n-p type and is so poled as to supply drive current for relay drive transistor 35.

This drive current is applied through resistor 41 to the emitter-base junction of relay drive transistor 35 to control the conduction of transistor 35 and thereby control the energization of a respective relay. The conduction of control transistor 39 is in turn controlled by a bias control network including resistors 42, 43, 44, and 45 and Zener diode 46 as activated by relayl selector switches 36, 37, 38, and relay contacts 31-1, 32-1, and 33-1. Diodes 47 through 52 serve as current blocking devices to prevent the energization of relays other than the relays selected for energization.

Before any one of relays 31, 32, or 33 has been selected to be energized, all of the relay selectorswitches 36, 37, and 38 are open, all of the relays are de-energized, and relay contacts 31-1, 32-1, and 33-1 are therefore also open. As a result bias resistors 43, 44, and 45 are not yet connected in a bias circuit of transistor 39. Transistor 39 is therefore in current saturation due to the large base current supplied from power source 30 through resistor 42. Part of the collector current of transistor 39 allows through resistor 41 and the base-emitter junction of transistor 35, thereby forward biasing transistor 35. Relay drive transistor 35 cannot yet conduct, however, because of its still open emitter-collector path.

In the interlock circuit of FIG. 2. any one particular relay may be selected to be energized by closing its corresponding relay selector switch. For instance, when it is desired to energize relay 31, relay selector switch 36 must be closed momentarily. The closing of relay selector switch 36 completes the power path for relay 31 which allows collector current of relay drive transistor 35 to ow, thereby energizing the relay. As soon as relay 31 energizes, its holding contact 31-1 closes, thereby bypassing relay selector switch 36 to keep relay 31 energized even after the release of momentary type selector switch 36.

Holding contact 31-1 and relay selector switch 36 perform, however, a dual function in this embodiment of the invention, in that they also control the bias of control transistor 39. That is, when relay selector switch 36 and later holding contact 31-1 are closed, they also complete the connection for resistor 43 through diode 50 into the bias circuit of transistor 39. Whereas before the closing of relay selector switch 36 the entire voltage of source 30 was applied through resistor 42 across the combination of the emitter-base junction of transistor 39 and Zener diode 46, after the closing of switch 36 this bias voltage is limited to substantially the voltage appearing across resistor 43. That is, resistors 42 and 43 now act as a voltage divider network to lower the forward bias applied to control transistor 39. As long as only one of the three bias resistors 43, 44, or 45 is connected in the bias circuit, however, the bias voltage applied to the base of transistor 39 is suiiicient to forward bias transistor 39; that is, the voltage across resistor 43, VMS, is greater than the sum of the voltage drops across the emitter-base junction of transistor 39, VBE39, and Zener diode 46, Vzis, 'S0 that VR43 VBE39+VZ46 Any one of the other two relays of the circuit of FIG. 2 may be selected in a similar marmer by momentarily closing the corresponding relay selector switch. If, for instance, relay 32 were to be energized, relay selector switch 37 would be closed, whereas relay selector switch 38 would be closed in order to energize relay 33. A particular relay that has previously been energized may be de-energized by interrupting its power path by a momentary opening of normally closed switch 34; either one of relays 31, 32, or 33 may be de-energized in this manner.

In the case where a relay that has been selected for energization is the first relay to be energized, that is, all of the relays are deenergized prior to the selection of this particular relay, the sequence and operation for any relay is identical to the sequence and operation described in connection with relay 31, except that holding contacts 32-1 and 33-1 function together with switches 37 and 38 and resistors 44 and 45 to energize relays 32 and 33, respectively. On the other hand, when an additional relay is being selected to be energized while one other relay is already energized, the following operating sequence takes place if it is assumed, for instance, that it is desired to energize relay 33 while relay 31 is already in an energized state. Because relay 31 is already energized, holding contact 31-1 is closed; control transistor 39 is still forward biased even though resistor 43 is connected in the bias circuit, and transistor 35 is conducting through the closed holding contact 31-1 to maintain relay 31 energized. Since it is now desired to energize relay 33 instead of relay 31, two functions are performed in sequence; namely, relay 31 is first de-energized, and then relay 33 is energized. Both of these operations are accomplished in the proper sequence -by closing relay selector switch 38. As soon as relay selector switch 38 is closed, resistor 45 is connected in parallel with resistor 43, thereby re` ducing the bias voltage on control transistor 39 to such an extent as to turn off transistor 39 instantaneously which, in turn, turns oi transistor 35. The result is fourfold and occurs in the following sequence: 1) even though relay selector switch 38 is closed, completing the power path for the coil of relay 33, relay 33 cannot yet energize because transistor 35 is turned olf; (2) since transistor 35 is turned off, relay 31 becomes de-energized, thereby opening its holding contact, where the opening of holding contact 31-1 disconnects resistor 43 from the bias network so that (3) control transistor 39 becomes again forward biased, thereby again turning on relay drive transistor 35, which allows (4) the energization of relay 33 through relay selector switch 38. Once relay 33 has become energized, holding contact 33-1 bypasses relay selector switch 38 to maintain the relay energized even after switch 38 has been opened again. Relay 33 remains energized until either switch 34 becomes momentarily opened or until another relay is selected.

Resistors 43, 44, and 45 are chosen to have such resistance values that when source 30 is applied through resistor 42 across the parallel combination of two or more of these resistors a voltage is developed across the parallel resistor combination which is smaller than the sum of the voltages across Zener diode 46 and the emitter-base junction of control transistor 39. That is, when at any one time more than two of the bias resistors 43, 44, or 45 are connected in parallel in the bias circuit, control transistor 39 is immediately turned off, thereby turning oi relay drive transistor 35 to positively prevent the energization of more than one relay at any one time. This feature of the invention is particularly valuable in circuit applications in which it is required that never more than one relay be energized at any one time in order to prevent undesired circuit operation or damage to the equipment.

The present invention further prevents the inadvertent selection of more than one relay at a time. This result is a necessary consequence of the bias network operation, since the simultaneous selection of two relays at any one time causes two bias resistors to be connected in parallel through their respective relay selector switch contacts which reduces the bias on control transistor 39 below the required forward bias level, thereby turning off relay drive transistor 35. As a result, no relay can energize as long as more than one relay selector lswitch is being activated at any one time. As soon, however, as only one relay selector switch remains activated, the relay corresponding to that one relay selector switch energizes.

The most significant feature of the interlock circuit of FIG. 2, however, is the requirement of only one holding contact per relay, which feature facilitates the extension of the interlock circuit of the instant invention to any nurnber of relays. The present invention provides, therefore, for a simple and efficient interlock circuit in which only one of any number of relays can be energized at any one time.

It is to be understood that the above-described arrangements are illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. In a relay interlock circuit comprising a plurality of relays each having an operating coil connected to at least one of its make contacts, a source of driving current having a rst terminal connected to each of said one make contacts and a second terminal, a relay drive transistor having a base electrode, an emitter electrode connected to,

said second terminal, and a collector electrode connected to each of said operating coils to form a plurality of series paths each including said source of driving current, the emitter-collector path of said relay drive transistor, and the said operating coil and said one make contact of an individual one of said relays, a plurality of switches for selecting an individual one of said relays for activation, an individual one of said plurality of switches being connected in parallel with each individual one of said make contacts, a voltage divider network comprising a first resistor connected to said second terminal and a plurality of second resistors each connected in series with said first resistor and an individual one of said make contacts across said source of driving current, a device for developing a reference voltage connected at one end to said rst terminal of said source of driving current, and control means connected to the other end of said reference voltage device, to the junction between `said iirst and second resistors and to the base electrode of said relay drive transistor to cause conduction through said relay drive transistor when the circuit through an individual one of said plurality of said second resistors is completed and to terminate conduction through said relay drive transistor When the circuit through more than an individual one of said plurality of said second resistors is completed, a rst and a second plurality of diodes for preventing current through unselected relays, an individual one of said rst plurality of diodes being serially connected with each of said second resistors between said first resistor and an individual one of said make contacts, and an individual one of said second plurality of diodes being serially connected with the operating coil of each individual relay between the relay make contact and the collector electrode of said relay drive transistor.

References Cited UNITED STATES PATENTS 3,311,795 3/1967 Gilbert 317-137 LEE T. HIX, Primary Examiner U.S. C1. X.R. 

